As a secondary battery, a lead secondary battery, a nickel-cadmium secondary battery, nickel hydride battery, or the like has been conventionally used. Recently, performance of electronics such as a mobile phone, a video camera, a notebook-size personal computer, and the like is remarkably improved, and a requirement for improving performance of a secondary battery as a power source of the aforementioned electronics has also been increased.
A lithium ion secondary battery which is a nonaqueous electrolyte secondary battery using a lithium compound as a positive electrode material together with a carbonaceous material as a negative electrode material can control growth of dendrite and powderization of lithium by utilizing charging and discharging lithium. For this reason, superior cycle life performance can be provided, and high energy densification and high capacitization can be achieved. As the aforementioned lithium compound, for example, a lithium transition metal oxide such as LiCoO2, LiNiO2, LiMn2O4 or the like is put to practical use.
However, the lithium ion secondary batteries use expensive metals as constituent elements of positive electrode materials, and many conventional positive electrode materials represented by LiCoO2, LiNiO2 or LiMn2O4 have a reduced amount, such as about 0.5, of a lithium atom capable of being reversibly occluded and released per one transition metal atom. Therefore, more effective utilization of a transition metal, as well as, development for a positive electrode material based on abundant and cheap elements have been desired.
In addition, conventional positive electrode materials have, in general, problems in performance stability and safety. In particular, sufficient stability with respect to cycle characteristics at high temperatures, storage properties, self-discharging properties and the like is not exhibited, and in addition, there is a problem in safety in which oxygen generates due to thermal decomposition, and thereby, ignition may be caused. It is believed that this may be caused by not only high reactivity between an electrode and an electrolyte in a lithium ion secondary battery due to high voltage, but also the crystal structure of the electrode material itself.
Therefore, studies for further improving properties as a positive electrode of a lithium transition metal oxide have been carried out by substituting a part of the transition metal of the aforementioned lithium transition metal oxide with a nonmetal element such as phosphorus, silicon, boron or the like. For example, Japanese Unexamined Patent Application, First Publication No. 2001-180939; WO 2005/99022; and WO 2005/99023 report a technology for improving cycle characteristics at high temperatures, storage properties, self-discharging properties, and the like of a secondary battery by substituting about 10 to 20% of a transition metal of LiMn2O4, LiCoO2 or the like with phosphorus, silicon, boron or the like.
However, in this case, for example, with respect to LiCoO2, it is reported that a single phase is formed when the amount of silicon is up to 10%, but another phase is formed when the amount is 35% (Solid State Ionics (2006), 177 (3-4), 317-322). It can be mentioned that the system in which the transition metal is substituted with a small amount of silicon is clearly different from a lithium transition metal silicate in which the mole ratio of a transition metal and silicic acid is around 1:1.
On the other hand, with respect to a lithium transition metal silicate in which the mole ratio of a transition metal and silicic acid is around 1:1, for example, a lithium transition metal silicate in which lithium silicate is used as a silicon source is reported (Electrochemistry Communications 7 (2005) 156-160). Usability thereof as a positive electrode of a so-called lithium ion secondary battery is suggested.
However, a lithium transition metal silicate in which the mole ratio of a transition metal and silicic acid is around 1:1 is, in general, synthesized by sintering an inorganic compound such as an inorganic lithium salt, a transition metal salt, silica or the like, as a raw material at high temperatures. For this reason, it is difficult to control crystallinity, particle size, particle size distribution, and the like of a product. The performance thereof is not necessarily desirable as a positive electrode material, compared to conventional lithium transition metal oxides. These materials basically have problems in that the capacity of reversibly occluding and releasing a lithium ion is reduced, and charge and discharge characteristics are degraded in accordance with repeating charging and discharging.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2001-180939    [Patent Document 2] WO 2005/99022    [Patent Document 3] WO 2005/99023    [Non-Patent Document 1] Solid State Ionics (2006), 177 (3-4), 317-322    [Non-Patent Document 2] Electrochemistry Communications 7 (2005) 156-160